The present invention relates to packaged respiring foodstuffs and improvements in the art of packaging foodstuffs which produce gas, particularly CO2 respiring foodstuffs, especially cheeses such as for example emmental, gouda and edam.
Many hundreds of different kinds of cheese are made today. The cheese making art is very old with evidence of cheese making as far back as 2300 B.C. Cheese is a cultured milk product i.e. typically a starter culture of bacteria which produce lactic acid as added to milk along with an enzyme called xe2x80x9crenninxe2x80x9d. Rennin typically comes from rennet from the stomach of a calf or lamb, but may be derived from either animal or plant sources. The acid produced by the bacteria alters the pH of the milk to an acidity which causes a milk protein termed xe2x80x9ccaseinxe2x80x9d to coagulate thereby forming curds. Rennin is an enzyme which facilitates curd formation. Typically, both acid produced by bacteria and rennin are used together to form cheese curds and whey. Curds aggregate holding fat and whey in a network of protein. In cheese making this curd formation is usually followed by pouring off the whey and concentration of the curds. To remove additional whey, curds may be cut, pressed, cooked and/or salted to produce what is termed xe2x80x9cgreenxe2x80x9d or unripened cheese. Here xe2x80x9cgreenxe2x80x9d refers to the youth or lack of aging of the cheese at this point in manufacture. The green cheese may then be aged or ripened for anywhere from a few days to up to four years or more depending upon the cheese variety. This ripening may continue even after packaging, but is generally slowed by holding cheese at lower refrigeration temperatures.
The above description relates to generally known processes for making natural cheeses. Also known are xe2x80x9cprocessedxe2x80x9d cheeses which are ground natural cheeses which typically mix unripened and ripened cheeses with other ingredients such as added milk and stabilizers followed by pasteurization and usually packaging while hot.
In forming natural cheeses, specific molds or bacteria may be added just prior to or during ripening to produce particular varieties of cheese having different characteristics such as flavors, aromas, textures and appearance.
For example, blue cheeses are made by inserting a blue green mold, Penicillium roquefort into the interior of the cheese. There are also surface ripened cheeses such as brie and camembert which have an exterior surface coat of a white mold Penicillium camembert. Cheeses such as brick and limburger are ripened by bacteria which are coated on the surface of the cheese. The original starter culture bacteria also may provide distinctive characteristics for ripening. Bacteria added in the starter culture is used for ripening in production of hard and semi-hard cheeses such as Parmesan, Cheddar and Gouda. Swiss type cheeses may also be ripened using the original starter culture, but typically additional bacteria such as Propionibacter Shermanii is added to form the xe2x80x9ceyesxe2x80x9d of the cheese. In emmental or swiss-type cheeses these xe2x80x9ceyesxe2x80x9d are formed as gas pockets of carbon dioxide (CO2) which is given off in large amounts by the bacteria which is nourished by lactic acid (which is produced by other bacteria in the starter culture). On grading of swiss-type cheese, cheese graders (which may be licensed by various governmental entities) consider the amount, size and development of eyes as well as the cheese appearance including uniformity of firmness, and its flavor and aroma, shape, freedom from mold, color, size and saltiness.
After ripening, or after molding and pressing (for starter culture ripened varieties of cheeses), cheeses are coated or packaged to prevent physical damage, moisture loss and spoilage (eg. by mite infestation or growth of undesirable molds or bacteria). Many packaging materials and preventive coatings are in use for contact with cheeses including: fat, cloth, wax, metal foils and plastic films and sheets. Waxes and resins in particular have been used for many years to coat dry, hard or semi-hard cheeses such as cheddar, cheshire, gouda, edam and danbo by dipping the cheese into melted wax. Cheese has also been packaged into polymer film under conditions which allow ripening of the cheese in the package.
In discussing plastic film packaging, various polymer acronyms are used herein and they are listed below. Also, in referring to blends of polymers a colon (:) will be used to indicate that the components to the left and right of the colon are blended. In referring to film structure, a slash xe2x80x9c/xe2x80x9d will be used to indicate that components to the left and right of the slash are in different layers and the relative position of components in layers may be so indicated by use of the slash to indicate film layer boundaries. Acronyms commonly employed herein include:
PExe2x80x94Polyethylene (an ethylene homopolymer and/or copolymer of a major portion of ethylene with one or more xcex1-olefins)
EVAxe2x80x94Copolymer of ethylene with vinyl acetate
PVDCxe2x80x94Polyvinylidene chloride (also includes copolymers of vinylidene chloride, especially with vinyl chloride)
EVOHxe2x80x94A saponified or hydrolyzed copolymer of ethylene and vinyl acetate
EAAxe2x80x94Copolymer of ethylene with acrylic acid
Various published patent documents disclose different types of cheese packages, packaging films and processes for packaging.
U.S. Pat. No. 1,925,443 (Gere) discloses flexible wrappers and a process for packaging uncured cheese wherein the cheese ripens or cures in the package. This patent states that xe2x80x9cThe package must be of moisture-proof and impervious material, and it must be so sealed as to exclude air, but at the same time, it must provide for the escape of excess carbon dioxide evolved in the course of fermentationxe2x80x9d. Preferred wrappers include xe2x80x9ccellulose viscosexe2x80x9d or xe2x80x9ccellulose acetatexe2x80x9d which may subsequently be coated with paraffin. Disadvantageously, manufacture of these films is complex, time consuming and expensive. Also, it is difficult to adjust CO2 permeabilities for use on different cheeses.
U.S. Pat. No. 2,494,636 (Stine) discloses a method of making emmental (swiss) cheese which comprises applying a coat of extensible, flexible, fluid proof sealing material to the exterior surface of the uncured cheese to seal the surface prior to eye development followed by curing under controlled pressure in an expandable mold. Suitable sealing materials are said to be wax, or a wrap of an elastic-flexible material such as cellophane, the inner surface of which may be coated with a flexible and elastic wax. The packaging materials disclosed here have the same disadvantages as described above for those materials disclosed in the Gere patent.
U.S. Pat. No. 2,871,126 (Smith et al.) discloses a method for manufacturing emmental type cheese which is also known as Swiss cheese. This patent refers to use of thermoplastic film as a moisture proof, fluid-proof material for wrapping the cheese after the brine step for curing in molds. A disadvantage of this disclosed film is that the moisture proof wrapper does not have an adjustable CO2 permeability.
U.S. Pat. No. 2,813,028 (Jackson, Jr.) discloses processes for producing Cheddar cheese. In one process green cheddar curd is extruded into preformed wrappers which may be made of cellulose based films such as cellophane, rubber chloride based films or polyvinylidene chloride based films such as saran. It is preferred that the films have the following characteristics:
(1) substantially moisture proof i.e. having relatively low moisture vapor transmission rate to prevent drying out
(2) slightly permeable to carbon dioxide to permit normal curing
(3) cling or stick to cheese to prevent mold growth
(4) slightly extensible to improve cling between wrapper and cheese by overfilling
(5) transparent or translucent to improve appearance.
The disclosed films suffer from disadvantageously, controlling CO2 permeability by slightly opening the ends of the package. This removes the physical, moisture and oxygen barrier at those openings thereby subjecting the cheese to the deleterious effects of excessive oxygen, loss of moisture and exposure to the environment.
Canadian Patent Application 2,053,707 (Mueller) discloses laminate films for packaging soft cheeses such as camembert and brie. Known materials for packaging such soft cheese is said to include polyethylenes with and without ethylene vinyl-acetate copolymers, polypropylenes, nylon/polyethylene laminates, and polyester/polyethylene laminates. Oxygen and carbon dioxide transmission rates are said to be xe2x80x9cof primary importance in the packaging of many soft cheeses, as well as other foods items which require a packaging material of high gas permeability such as many fruits and vegetablesxe2x80x9d. (See page 1). The disclosed film of Mueller comprises a first film component (which is perforated) laminated to a gas permeable layer which include at least one layer comprising butadiene styrene copolymers. Relative gas and moisture transmission rates are said to be determined by the size and number of perforations in the first layer as well as the thickness and permeability of the second layer.
In the examples, permeabilities of the film of Example 3 are stated as follows:
xe2x80x9cThe water vapor transmission rate averaged about 2.73 g/100 in2, 24 hr. at 100xc2x0 F. and 100% RH. The oxygen transmission rate averaged about 4858.9 cm3/m2 atm., 24 hrs. at 73xc2x0 F. The carbon dioxide transmission rate averaged about 30204.0 cm3/m2, atm., 24 hrs. at 73xc2x0 F.xe2x80x9d
These films have a very high permeability to oxygen as well as carbon dioxide and such extremely high oxygen permeability while perhaps suitable for mold cured cheeses is undesirable for hard or semi-hard cheeses such as emmental, Gouda, Edam and the like due to the possibility of facilitating undesirable mold growth.
Canadian Patent Application No. 2,050,837 (Gillio-Tos et al.) discloses polymer mixtures of polyvinylidene chloride and polyethyloxazoline which are purportedly useful in forming monolayer or multilayer films having increased moisture permeability with no substantial change in permeability to oxygen or carbon dioxide. This combination of properties purportedly is xe2x80x9cindicative of utility in packaging, for example, medical applications, casings and the curing of non-gassing cheeses such as Parmesanxe2x80x9d (page 3, last paragraph). A table shows moisture, oxygen and carbon dioxide permeability rates. These films are made from chlorinated polymers which are increasingly more difficult to dispose of or recycle as further discussed below.
EP 457 598 (Shah et al) discloses a polyamide based multilayer film for packaging cheese. This polyamide film is disclosed as having xe2x80x9can oxygen transmission rate of no more than 500 cc/M2, 24 hrs., atm and a carbon dioxide transmission rate of at least 750 cc/m2, 24 hrs., atm.xe2x80x9d. Example 5 purportedly discloses a 1 mil (25.4 micron) thick biaxially oriented film having a core layer comprising a blend of about 70% EVOH and about 30% of a polyamide in combination with polypropylene or propylene copolymer based outer layers and this film has a reported shrinkage at 220xc2x0 F. (104xc2x0 C.) of 24% in two directions. The core layer is about 14% of the thickness of the film which would be 0.14 mil (3.6 microns). Example 8 purportedly had outer layers of 90% linear medium density polyethylene blended with 10% of an EVA-based masterbatch and a core layer which was a blend of 70% nylon and 30% EVOH, with the core layer comprising 25% of the total film thickness. Disadvantageously, the shrinkage values of this film are achieved at high temperatures with the lowest reported measurement being made at 104xc2x0 C. as denoted in the Table on page 8, and it is therefore to be expected that the shrinkage values at 90xc2x0 C. and lower temperatures would disadvantageously be much less. This results in films having high shrinkage only at undesirably high temperatures.
Various monolayer and multilayer thermoplastic films have been commercialized for packaging cheeses. Three to five layer films are common. Typical structures include: EVA/PVDC/EVA, EVA/EVA/PVDC/EVA, Ionomer/EVA/PVDC/EVA, and variations thereof where ethylene based polymers are blended into one or more of the EVA layers. Some cheese packaging films are heat shrinkable at 90xc2x0 C. and others are not. Some of the nonshrinking films have an oxygen barrier comprising one or more layers of nylon or EVOH or a blend of EVOH with nylon. Such known nonshrinking films include structures of the type EVA:PE/Nylon, EVA: PE/Nylon/EVOH/Nylon/EVA:PE, EVA:PE/PVDC/Nylon, EVA: PE/EVOH/Nylon, and EVA:PE/Nylon/EVA. The known nonshrinking EVOH containing films generally have a relatively thick EVOH containing layer, generally greater than 0.5 mil (12.7 microns).
Of the foregoing nonshrinking films, those containing EVOH have a typical oxygen permeability of less than 10 cm3 per m2 at 1 atm, 0% relative humidity and 23xc2x0 C. and are considered high barrier films. The terms xe2x80x9cbarrierxe2x80x9d or xe2x80x9cbarrier layerxe2x80x9d as used herein mean a layer of a multilayer film which acts as a physical barrier to gaseous oxygen molecules. Physically, a barrier layer material will reduce the oxygen permeability of a film (used to form the bag) to less than 70 cm3 per square meter in 24 hours at one atmosphere, 73xc2x0 F. (23xc2x0 C.) and 0% relative humidity. These values should be measured in accordance with ASTM standard D-1434.
Also known are films suitable for packaging cheese that are heat shrinkable at 90xc2x0 C. which contain nylon or a blend of EVOH and nylon. Axially stretched, especially biaxially stretched, films which are xe2x80x9cheat shrinkablexe2x80x9d as that term is used herein have at least 10% unrestrained shrinkage at 90xc2x0 C. (10% in both the machine direction (M.D.) and transverse direction (T.D.) for biaxially stretched films). Such known films include structures of the following types: Ionomer/PE/Nylon, Ionomer/EVA/Nylon, EAA/Nylon:EVOH/Ionomer, and PE/EVOH:Nylon/PE. Some of these EVOH containing heat shrinkable films have an oxygen permeability in the high barrier range. A few heat shrinkable, EVOH-containing films have permeabilities which are outside the high barrier range such as e.g. about 30-35 cm3/m2 or even as high as 150-170 cm3/m2 at 1 atm, 0% relative humidity and 23xc2x0 C.
As shown in the present specification, high barrier film (whether shrinkable or not) which are very good oxygen barriers typically also have very low carbon dioxide permeabilities which may be disadvantageously low for packaging respiring articles such as cheeses, particularly hard and semi-hard cheeses. Packaging films which have low permeability to CO2 are subject to pillowing when hermetically sealed around an enclosed respiring article. If the respiration rate of the enclosed article exceeds the CO2 transmission rate for permeating the enclosing film, xe2x80x9cpillowingxe2x80x9d will occur. Pillowing or xe2x80x9cballooningxe2x80x9d refers to the inflation of the sealed film which typically causes the film surface to move away and out of contact with much of the surface of the enclosed article. For such respiring articles as foodstuffs e.g. hard and semi-hard cheeses, it is perceived that some customers view pillowing as a defect and avoid purchase of refrigerated foodstuffs having a pillowed container. Furthermore, it is believed that retention of high concentrations of CO2 about a respiring foodstuff may possibly adversely affect the curing process itself, possibly delaying development of the desirable characteristics of the microbiological processes including e.g. full flavor and aroma development.
Also, the prior art EVOH-containing high permeability cheese films have several disadvantages for packaging respiring cheeses including one or more of the following: undesirably low shrink values particularly at low temperatures e.g. 90xc2x0 C. or lower, an undesirably narrow heat sealing range, use of expensive resins such as ionomer in the other layers, and poor optical properties such as high haze, low gloss and/or streaks or lines which detract from the film appearance. Furthermore, known EVOH-containing cheese films have a disadvantageously thick EVOH-containing layer which is often 2 to 10 times thicker than the present invention and difficult to process into an oriented film, difficult to make into a heat shrinkable film having high shrinkage values and shrink forces, especially in two directions, and which requires more material which may increase expense.
As shown by the above, many different multilayer film structures have been and continue to be commercially made and used to package cheeses. These structures all suffer from various disadvantages, especially with respect to packaging xe2x80x9crespiringxe2x80x9d cheeses i.e. those cheeses which give off CO2 gas.
For example, higher levels of CO2 permeabilities with PVDC containing films require that the PVDC layer be heavily plasticized to achieve gas permeability. Such plasticizers may adversely affect other film properties including processability, optical properties, and orientability.
Also, recycling of PVDC polymers is difficult, particularly where the waste polymer is mixed with other polymers having different melting points. Attempts to remelt film containing PVDC frequently results in degradation of the PVDC component.
It is an object of the invention to provide a multilayer film having a high carbon dioxide permeability and relatively low oxygen permeability which is controlled by a thin core layer.
It is still another object of the invention to provide a film having low permeability to water vapor.
It is another object of the invention to provide a multilayer film containing EVOH and having high shrinkage values at 90xc2x0 C. or less.
It is a further object of the invention to provide an irradiatively crosslinked, CO2 permeable multilayer film having an EVOH:nylon blend core layer having a broad impulse heat sealing voltage range.
It is yet another object of the invention to provide an EVOH containing multilayer film having good optical properties.
It is a further object of the invention to provide a chlorine-free packaging film.
It is an object of the invention to provide a film for packaging respiring articles, particularly cheeses, which have reduced pillowing of the package after vacuum packaging.
It is another object of the invention to provide a packaged cheese using a multilayer film having a thin oxygen permeability retarding layer which is also permeable to carbon dioxide.
The above and other objects, benefits and advantages of the invention will be apparent from the disclosure below which is exemplary and nonlimiting. It is not necessary that each and every object listed above be found in all embodiments of the invention. It is sufficient that the invention may be usefully employed.
According to the present invention a gas releasing foodstuff especially CO2 respiring cheese, is packaged in a multilayer, thermoplastic, flexible film of at least five layers having a thin core layer (about 0.05 to-0.1 mil (1.7-2.54 microns) which comprises a blend of about 20-70 weight percent nylon and about 30-80% of an ethylene vinyl alcohol copolymer having an ethylene content of about 38 mole percent or higher. Preferably the inventive film will be heat sealable having at least one layer which is crosslinked, preferably by irradiation. In a preferred highly useful embodiment of the invention, the film will be heat shrinkable at temperatures such as 90xc2x0 C. or lower, and may have shrinkage values in one or both of the MD and TD directions of at least about 20%, and advantageously e.g. for packaging cheese may be at least 30%.
In one embodiment of the invention, a packaged respiring natural cheese such as emmental, Jarlsberg, edamer, butterkase, Gouda or Edam is provided where an EVOH:nylon 6/66 copolymer (here 6/66 is used to denote a copolymer of nylon 6 and nylon 66) blend core layer of the film has the relative amounts of nylon and EVOH adjusted to provide the desired level of CO2 permeability and O2 barrier properties. Such a film need not be perforated, and preferably is unperforated, yet a high level of CO2 permeability may be obtained without perforations.
The inventive film, bag, process and package of the present invention may be used as a CO2 permeable, oxygen barrier film for holding a respiring natural cheese during curing or for packaging for sale of such a cheese after the predetermined curing period. After curing of a cheese such as emmental, the cheese which may be a large block of up to forty pounds (18.2 Kg) or more is often cut up into smaller sizes such as 10 or 7 pound (4.5-3.2 Kg) portions or less for sale to commercial establishments such as hotels, restaurants or other institutions or for retail sale through deli counters or to individual consumers. Forty pound (18.2 Kg) blocks of cheese are frequently packaged in thermoplastic bags having a flat width of 18-22 inches (46-56 cm) while smaller weights are typically packaged in bags having a smaller flatwidth e.g. of less than 10 inches (25.4 cm) for weights of 5 pounds (11 Kg) or less. The present invention may be employed as bags in the various typical sizes. By xe2x80x9cflatwidthxe2x80x9d is meant the transverse width of a flattened tubular film. The flatwidth is also equal to xc2xd of the circumference of the tubular film.
The invention has particular utility in packaging natural cheese. Cheese as it is produced from milk with curds that are only cut or pressed is said to be natural (as further discussed above) and may be contrasted with xe2x80x9cprocessxe2x80x9d cheese which is made from natural cheese e.g. by grinding, heating and pasteurizing natural cheese with additives which may include milk, water, emulsifiers and/or preservatives. Pasteurization stops or inhibits the aging/ripening process which gives off CO2. Therefore, the CO2 permeable films of the instant invention are particularly advantageous for enclosing natural respiring cheeses because the inventive films allowed CO2 to escape by permeation across the film wall. At the same time the present films and bags made therefrom are much less permeable to oxygen and this is an advantage because the presence of large amounts of oxygen is believed to facilitate growth of undesirable molds.
The present invention is particularly well adapted to packaging respiring cheeses, especially cheeses having eyeholes. Eyeholes in cheese are produced by pockets of carbon dioxide (CO2) which is generated by CO2 producing bacteria such as Propionibacter shermanii.
The invention may be suitably employed with hard cheeses including those having eyeholes which are typically round such as Emmental, Jarlsberg, Gruyere, herregaardsost, danbo, asiago, viereckhartkase, bergkase and samsoe, as well as those cheeses which typically have irregular holes such as Cheshire, maribo, svecia and manchego, and those cheeses which generally have no or few holes such as Cheddar, and provolone. The invention may also be suitably employed with semi-hard cheeses including those having small eyeholes such as Gouda, Edam, fontina, raclette and those typically having irregular eyeholes such as trappist, tilsit and havarti, and even those which typically have no holes such as butter cheese (butterkase), cantal, St. Paulin and feta.
Advantageously, the relative amounts of EVOH polymer and nylon polymer may be adjusted according to the present invention to provide films, bags and packages having various permeabilities to gases including CO2. An example of suitable CO2 permeabilities for various cheeses is given in Table A below.
Embodiments of the present invention for use in high CO2 permeability applications will generally use a core layer having a greater amount of nylon ( greater than 50 to 70%) and lesser amounts of EVOH copolymer (30 to  less than 50%) to produce a film having higher CO2 permeability. The high permeability film will also utilize an EVOH having an ethylene content of about 48 mole percent or higher because EVOH copolymers having lower ethylene contents have been found to produce films which are more gas impermeable. Preferably the oxygen permeabilities of the high CO2 permeability films and bags of the present invention will be less than 800 cm3/m2 and greater than 500 cm3/m2 at 24 hours, 1 atmosphere, 0% relative humidity and room temperature (20-23xc2x0 C.).
Embodiments of the present invention for use in low CO2 permeability applications will generally use a core layer having a greater amount of EVOH (70-80%) and lesser amounts of nylon (20-30%) to produce a film having a low CO2 gas transmitting rate, particularly when using an EVOH copolymer having an ethylene content of about 48 mole percent. However, EVOH copolymers having lower contents of ethylene e.g. of 44 mole % or as low as 38 mole % may be employed to make low CO2 permeability films which are suitable for packaging articles which respire low levels of CO2 such as gouda, edamer, edam, butterkase and cheddar cheeses, by blending lower amounts of EVOH with higher amounts of nylon. The appropriate blend proportions to achieve the desired level of gas permeability may be determined in view of the present specification without undue experimentation. Even these low CO2 permeability films made according to the present invention will have an oxygen permeability of about 40 cm3/m2 or higher at 24 hours, 1 atmosphere, 0% relative humidity and room temperature, preferably the oxygen transmission rate will be less than about 75 cm3/m2.
Embodiments of the present invention for use in medium CO2 permeability applications to adjust and control the CO2 permeability of the film to a level between that of high and low CO2 permeable films of the invention will generally use nylon and EVOH in amounts between those used for high and low permeability films i.e.  greater than 30-50 nylon blended with 50 to  less than 70% EVOH when using an EVOH having an ethylene content of about 48 mole %. As noted above, use of EVOH resins having a lesser ethylene content e.g. of about 38 mole % or 44 mole % may require additional nylon to achieve the desired permeability. It should be apparent from the above that use of EVOH resins containing an ethylene content greater than 48 mole % may require less nylon to achieve equivalent permeabilities relative to the 48 mole % EVOH blends discussed above. For these medium CO2 permeability films, the oxygen transmission rate is also 40 cm3/m2 or higher at 24 hours, 1 atmosphere, 0% relative humidity and at room temperature (about 20-23xc2x0 C.).
In addition to changing the proportion of the blended amounts of EVOH and nylon in the core layer to adjust the gas e.g. CO2 permeability of the films of the invention, the thickness of the core layer may also be varied from about 0.05 to about 0.10 mils (1.3-2.54 microns). Also, while it is preferred that the core layer consist essentially of nylon and EVOH, the present invention recognizes the possibility that the other additives including polymers e.g. other nylons may be blended into the core layer to purposefully affect core layer properties such as gas permeability or moisture resistance.
Advantageously, the present invention permits ripening of CO2-producing cheeses in a thermoplastic multilayer film having a thin core layer of a blend of EVOH with nylon with little or no weight loss during ripening. The moisture barrier properties of the film minimize weight loss from moisture permeation through the film after packaging. Films of the present invention having water vapor transmission rates less than 30 grams per square meter per 24 hours at 100xc2x0 F. (37.8xc2x0 C.) under ambient pressure at (xcx9c1 atmosphere) have been found to have desirably low weight loss from moisture permeation through the film.
Also, the oxygen barrier properties of the inventive film reduces or eliminates losses of cheese caused by scraping off mold from contaminated surfaces before repackaging for retail sale. Product losses and sensory defects due to mite infection and mold growth are also prevented by use of the film according to the present invention. The present invention may be beneficially used as a ripening film particularly for rindless cheese where rinds having surface molds or bacteria to give particular flavor and odor sensory characteristics to the cheese are not employed. The inventive films and bags are particularly useful for packaging cheese, but may also be employed as packaging for a wide variety of food and non food articles.
Some of the benefits of the inventive film include: relatively low permeability to oxygen and water vapor, particularly in combination with higher CO2 permeability; controlled permeability (and high permeability if desired) to carbon dioxide without perforations in the film; resistance to degradation by food acids, salts and fat; high shrinkage values at low temperatures (90xc2x0 C. or lower); residual shrink force which maintains a low level of oxygen contact with the food surface after opening; good heat sealability especially over a broad voltage range on commercial sealers; low levels of extractables with compliance with governmental regulations for food contact; delamination resistance; low haze; high gloss; easy to remove from an enclosed foodstuff such as cheese; does not impart off tastes or odors to packaged food; good tensile strength; a surface which is printable; and good machinability.
Advantageously, a preferred embodiment of the invention has a high CO2 permeability at 5xc2x0 C. with relatively low O2 and low water vapor permeabilities in combination with good low temperature (90xc2x0 C. or less) shrinkage values. In an especially preferred embodiment the inventive film has at least 20% (more preferably about 30% or higher) shrinkage values in at least one direction and preferably two directions at 90xc2x0 C. or less. Also, preferred films are heat sealable over a broad voltage range, and preferably heat shrinkable at low temperatures in combination with such broad range heat sealability.
The invention in all of its embodiments comprises or utilizes a multilayer thermoplastic polymeric flexible film of 10 mils (254 microns) or less having an unusually thin core layer containing a blend of EVOH and nylon. This EVOH:nylon blend layer controls the gas permeability of the film. Such films will preferably have a thickness of about 2-2.5 mils (50.8-63.5 microns) although suitable films e.g. for packaging foodstuffs as thick as 4 mils (101.6 microns) or as thin as 1 mil (25.4 microns) may be made. Typically films will be between about 1.5-3 mil (38.1-76.2 microns). Especially preferred for use as bags or films for packaging articles including foodstuffs, e.g. cheeses, are films wherein the multilayer film has a thickness of between about 2 to 2.5 mils (50.8-63.5 microns). Such films have good abuse resistance and machinability. Films thinner than 2 mils are more difficult to handle in packaging processes. Advantageously, preferred films are heat shrinkable. Preferred films may also provide a beneficial combination of one or more or all of the properties including high gloss, high shrinkage values at 90xc2x0 C. or less, good machinability, good mechanical strength and good relatively low oxygen barrier and water barrier properties with desirably high CO2 permeabilities.
The expression xe2x80x9cethylene vinyl acetate copolymerxe2x80x9d (EVA) as used herein refers to a copolymer formed from ethylene and vinyl acetate monomers wherein the ethylene derived units (monomer units) in the copolymer are present in major amounts (by weight) and the vinyl acetate derived units (monomer units) in the copolymer are present in minor, by weight, amounts.
The expression very low density polyethylene (xe2x80x9cVLDPExe2x80x9d) sometimes called ultra low density polyethylene (xe2x80x9cULDPExe2x80x9d), refers to linear polyethylenes having densities below about 0.915 g/cm3 and according to at least one manufacturer, possibly as low as 0.86 g/cm3. This expression does not include ethylene alpha olefin copolymers of densities below about 0.90 g/cm3 with elastomeric properties and referred to as elastomers. Some elastomers are also referred to by at least one manufacturer as xe2x80x9cethylene alpha olefin plastomersxe2x80x9d, but other manufacturers have characterized VLDPE as an ethylene xcex1-olefin with plastomeric properties. However, as hereinafter explained, ethylene alpha elastomers or olefin plastomers may be advantageously used in the practice of this invention as a minor constituent in certain layers of this multilayer film. VLDPE does not include linear low density polyethylenes (LLDPE) which have densities in the range of 0.915-0.930 gm/cm3. VLDPE""s as the term is used herein may be made by solution or fluidized bed processes using a variety of catalysts including Ziegler-Natta or metallocene catalysts.
VLDPE comprises copolymers (including terpolymers) of ethylene with alpha-olefins, usually 1-butene, 1-hexene or 1-octene, and in some instances terpolymers, as for example of ethylene, 1-butene and 1-hexene. A process for making VLDPEs is described in European Patent Document publication number 120,503 whose text and drawing are hereby incorporated by reference into the present document.
As for example described in Ferguson et al. U.S. Pat. No. 4,640,856 and Lustig et al. U.S. Pat. No. 4,863,769, VLDPEs are capable of use in biaxially oriented films and have superior properties to comparably made films having LLDPEs. These superior properties include higher shrink, higher tensile strength and greater puncture resistance.
Suitable VLDPEs include those manufactured by Dow Chemical Company, Exxon Chemical Company and Union Carbide Corporation.
EVOH is prepared by the hydrolysis (or saponification) of an ethylene-vinyl acetate copolymer, and it is well known that to be an effective oxygen barrier, the hydrolysis-saponification must be nearly complete, ie. to the extent of at least 97%. EVOH is commercially available in resin form with various percentages of ethylene and there is a direct relationship between ethylene content and melting point.
In the practice of this invention, the EVOH component of the core layer has a melting point of about 175xc2x0 C. or lower. This is characteristic of EVOH materials having an ethylene content of about 38 mole % or higher. EVOH having an ethylene content of 38 mole % has a melting point of about 175xc2x0 C. With increasing ethylene content the melting point is lowered. Also, EVOH polymers having increasing mole percentages of ethylene have greater gas permeabilities. A melting point of about 158xc2x0 C. corresponds to an ethylene content of 48 mole %. Preferred EVOH materials will have an ethylene content of 48 mole %. EVOH copolymers having higher ethylene contents may be employed and it is expected that processability and orientation would be facilitated, however gas permeabilities, particularly with respect to oxygen may become undesirably high for certain packaging applications which are sensitive to mold growth in the presence of oxygen.
It has been discovered that the inventive film with all of its advantages can only employ nylon 6/66 as the polyamide in the polymer blend of the oxygen barrier layer. Nylon 6/66 is a copolymer of nylon 6 and nylon 66. Nylon 6 is polyepsilon caprolactam. Nylon 66 is the polymer derived from adipic acid and hexamethylene diamine.
Nylon 6/66 is manufactured by different companies, in some instances with different percentages of the two monomers, possibly by different methods and presumably with different operating parameters. Accordingly, the properties of various nylon 6/66 copolymers may differ significantly. For example, the melting temperature decreases as the nylon 66 content is increased from 5% to 20 mole %.
When other nylons such as type 6,12 are used as the polyamide in the polymer blend of the oxygen barrier layer, numerous gels develop in the core layer of the five layer film and in some instances cracks develop. The gels may be due to EVOH-nylon 6,12 incompatibility or chemical reaction between the two polymers. The cracks probably develop because the polymer blend is not stretching uniformly during the orientation. These numerous gels and cracks are unacceptable in films for commercial use to package foodstuffs and indicate potential weak spots in the film integrity and permeability properties.
A preferred nylon is a nylon 6/66 copolymer having a melting point of about 195xc2x0 C., which has a reported nylon 6 component content of about 85 mole % and a nylon 66 component content of about 15 mole % and which is commercially available from Allied Chemical Co. of Morristown, N.J., U.S.A. under the trademark Nylon 1539.
It has been discovered that a workable film may be obtained by substituting nylon 11 (poly xcfx89-aminoundecanoic acid) for the nylon 6/66 copolymers in the present invention. Such film has many of the advantages of the present invention including good low temperature shrink values and desirable CO2 and O2 permeabilities, but the optical properties are not as excellent and noticeable lines may appear in the film, notwithstanding desirably low haze and high gloss values.
Advantageously, films of the present invention may have low haze e.g. less than 10 and preferably less than 5%, and high gloss e.g. greater than 65 Hunter Units (H.U.) and preferably greater than 75 H.U.
The inventive article is preferably a heat shrinkable multilayer film which must have at least five layers. These five essential layers are termed the first outer layer, the first adhesive layer, the core layer, the second adhesive layer, and the second outer layer. The first outer layer and second outer layer are disposed on opposing sides of the core layer and are attached thereto by the first and second adhesive layers, respectively. These five layers are essential to the film of this invention. When the film is in tube or bag form these layers comprise the wall of the tube or bag. This wall in cross-section has the first outer layer comprising an inner layer disposed closest to the tube""s (or bag""s) interior surface with the second outer layer disposed closest to the tube""s (or bag""s) exterior surface.
It is contemplated according to the present invention that tubular films having more than five layers may be constructed and that such additional layers may be disposed as additional intermediate layers lying between the core layer and either or both of the inner and outer layers, or these additional layers may comprise one or more surface layers and comprise either or both the interior or exterior surface of the tube. Preferably, the first outer layer will comprise the inner or interior surface layer of the tube where in use it will contact a foodstuff encased by the tube. Beneficially, this first outer layer will be heat sealable to facilitate formation of bags and hermetically sealed packages. Advantageously, the first outer layer as the interior or inner surface layer will, when used to package foodstuffs, be suitable for contact with foodstuffs containing protein, water and fat without evolving or imparting harmful materials, off tastes or odors to the foodstuff. In a preferred embodiment, the invention provides a film suitable for packaging cheeses, particularly cheeses which give off carbon dioxide gas (also termed xe2x80x9crespiringxe2x80x9d) while packaged, such as Emmental (swiss), Gouda or Edam. Beneficially, in the present invention the first outer layer may be the interior surface layer and may consist essentially of an ethylene vinyl acetate copolymer such as an EVA having about 10.5% by weight of vinyl acetate (10.5% VA) which facilitates orientation to produce a film having high shrinkage values especially at 90xc2x0 C. or lower (e.g. 85xc2x0 C. or 80xc2x0 C.). Advantageously, the heat sealing layer and indeed the entire film may be free of ionomer polymer yet provide entirely satisfactory performance without the added expense of using costly ionomer resin. If desired, an ionomeric resin may be used either alone or blended in one or more of the layers but such use is unnecessary to produce a film suitable for packaging respiring cheeses.
Also, it is preferred that the second outer layer will comprise the exterior surface of the tube or bag. As the exterior surface layer of the tube or bag, the outer layer should be resistant to abrasions, abuse and stresses caused by handling and it should further be easy to machine (i.e. be easy to feed through and be manipulated by machines e.g. for conveying, packaging, printing or as part of the film or bag manufacturing process). It should also facilitate stretch orientation where a high shrinkage film is desired, particularly at low temperatures such as 90xc2x0 C. and lower.
Advantageously, either or both of the first and second outer layers may be predominantly comprised of ethylene homopolymers or copolymers having at least 50% or higher ethylene content and may also be free of polypropylene or propylene copolymers having a propylene content of 50% or more, and films made with such outer layers according to the present invention may be oriented either uniaxially or biaxially by axial stretching at temperatures low enough to produce low temperature high shrink films. Such heat shrinkable films will have at least 10% shrink in at least one direction at 90xc2x0 C., but preferably will have at least 20% shrink at 90xc2x0 C. in at least one direction (preferably both directions) and advantageously may have at least 30% shrink at 90xc2x0 C. in at least one direction, but preferably both M.D. and T.D. directions, and beneficially may have at least 15% (more preferably at least about 20%) shrink at 80xc2x0 C. in at least one and preferably both M.D. and T.D. directions. The outer layers function to protect the core layer from abuse and may also protect it from contact with moisture which may impact or alter the gas barrier properties of the core layer EVOH and/or nylon.
Beneficially, in the present invention there are intermediate adhesive layers on either side of the core layer with these intermediate adhesive layers adhering the core layer to both the inner and outer layers. In a preferred embodiment the EVOH:nylon blend core layer directly adheres to the first and second adhesive layers which in turn are directly adhered respectively to the inner and outer layers. In a most preferred embodiment the film article consists essentially of five polymeric layers viz the inner layer, the first adhesive layer, the core layer, the second adhesive layer and the outer layer. This preferred embodiment provides a desirable combination of properties such as low moisture permeability, relatively low O2 permeability in combination with relatively high CO2 permeability, high gloss, good mechanical strength, chlorine-free construction, and desirable shrink forces in a low temperature heat shrinkable, multilayer packaging film which is delamination resistant and can be oriented without requiring addition of processing aids or plasticizers to the EVOH:nylon core layer. Preferably the core layer will be free of such processing aids or plasticizers.
Typical layer thicknesses for the inventive heat shrinkable film may be about 5-70% first outer (or inner) layer, 2-10% first adhesive layer, 2-10% core layer, 2-10% second adhesive layer and 20-35% second outer layer, although films with differing layer ratio thicknesses are possible. The function of the first outer layer (which is typically the inner layer in a tubular construction) is to provide a layer which has high shrinkage capability and a surface which is heat sealable to itself (or to the second outer layer where a lap seal is desired) on commercially available equipment and (for food packaging) to provide a hygienic surface for contact with the foodstuff which is typically a cheese, such as a semi-soft or semi-hard or hard cheese and especially a CO2 respiring cheese such as Edam, Gouda or Emmental (Swiss). In the present invention, to fulfill the second and third functions the thickness of the inner layer need not be great, but for shrinkability and ease of processing this layer will preferably be the thickest of the layers and in a preferred embodiment the major thickness ( greater than 50%) of the entire film. Alternatively, another layer may be the shrink controlling layer or a shrink controlling layer may be added on either side of the core layer. In such alternative embodiments the first outer layer for sealing and food contact may be made very thin i.e. 5% or less of the total thickness. It is important that this heat sealable layer be continuous, e.g. over the inner surface of the tube, and that it be extruded at a sufficient thickness to allow heat sealing (if desired) as well as being thick enough to accommodate the desired degree of stretching without bursting or failure.
Preferably, the first outer layer is a heat sealing layer which allows the film to be formed into bags. By the term xe2x80x9cheat sealing layerxe2x80x9d is meant a layer which is heat sealable to itself, i.e., capable of fusion bonding by conventional indirect heating means which generate sufficient heat on at least one film contact surface for conduction to the contiguous film contact surface and formation of a bond interface therebetween without loss of the film integrity. Advantageously, the bond interface must be sufficiently thermally stable to prevent gas or liquid leakage therethrough when exposed to above or below ambient temperatures during processing of food within the tube when sealed at both ends, i.e., in a sealed bag form. Finally, the bond interface between contiguous inner layers must have sufficient physical strength to withstand the tension resulting from stretching or shrinking due to the food body sealed within the tube.
The first outer layer especially as the inner layer of a tube according to the present invention also provides good machinability and facilitates passage of the film over equipment (e.g. for inserting foodstuffs such as cheese). This layer may be coated with an anti-block powder. Also, conventional antiblock additives, polymeric plasticizers, or slip agents may be added to the first outer layer of the film or it may be free from such added ingredients. In one embodiment of the invention the first outer layer consists essentially of an EVA copolymer.
Advantageously, the core layer functions as a controlled gas barrier, and provides the desired CO2 and O2 permeabilities for the article (e.g. foodstuff) to be packaged. It should also provide good optical properties when stretch oriented, including low haze and a stretching behavior compatible with the layers around it for ease of orientation. It is essential that the thickness of the core layer be less than about 0.10 mil (2.54 microns) and greater than about 0.05 mil (1.27 microns) to provide the desired combination of the performance properties sought e.g. with respect to carbon dioxide permeability, oxygen permeability, shrinkage values especially at low temperatures, ease of orientation, delamination resistance and cost. Preferably, the thickness of the core layer will also be less than about 5% of the total thickness of the multilayer film suitable thicknesses are less than 10% e.g. from 2 to 10% of the total film thickness.
The core layer must be a blend of EVOH and nylon which contains about 20-70 wt. % of nylon and about 30-80 wt. % of EVOH copolymer. Use of lower amounts of nylon (less than 20% and particularly less than 15%) results in a tendency for the core layer to crack (sometimes referred to as xe2x80x9cline-drawingxe2x80x9d) rather than stretch uniformly during orientation. This is in part due to the relatively brittle nature of EVOH polymers compared to e.g. EVAs and PEs. Similarly, at EVOH levels above 80% the tendency for such cracking, and formation of optical defects and potential core layer discontinuities is undesirably greater. At EVOH levels below 30% the oxygen gas permeability of the film tends to become undesirably excessive which may lead to an enhanced probability of mold growth due to increased oxygen permeability e.g. on cheeses packaged in a film of such construction. Preferably a high to medium high CO2 permeable film will have an EVOH:nylon content between about 30:70 percent and 60:40 percent while a medium low to low CO2 permeable film will have an EVOH:nylon content of between about 60:40 percent and 80:20 percent.
The outer layer provides mechanical strength, abrasion resistance and resists burn through during heat sealing. This outer layer is typically sufficiently thick to provide support and impart strength to the packaging film wall in order to withstand the shrinking operation, handling pressures, abrasion, and packaging with a foodstuff such as cheese. Advantageously, it may comprise a polyethylene i.e. an ethylene homopolymer or a copolymer of ethylene with a minor proportion of one or more alpha-olefins, which may provide a water vapor barrier which resists moisture permeation. High moisture barrier properties are desirable to avoid weight loss and undesirable drying of the cheese which may deleteriously affect the desired cheese sensory properties including texture, mouth feel, taste and appearance.
The multilayer film of the invention may be made by conventional processes including e.g. slot cast or blown film processes, but preferably will be made by an orientation process, especially under conditions to produce a film which is heat shrinkable at 90xc2x0 C. or less. Nonshrink films according to the present invention may be used as overwraps, stretch wraps or as industrial plastic wrap. Shrink films according to the present invention may be used in value added applications. For example, a packaged foodstuff such as cheese having a heat shrinkable film enclosure according to the invention will advantageously cling to the foodstuff even after opening. Non-shrink bags have a tendency to fall away from the sides of the enclosed product (e.g. cheese) once the vacuum is broken by either intentional or accidental opening. Once the film separates from the enclosed article surface, oxygen comes into contact with the article surface and product defects on susceptible products such as growth of undesired mold on cheese may occur. Many prior art films and bags are nonshrink bags which suffer from this defect thereby causing spoilage and waste when used to package perishable foodstuffs such as cheese. Undesired mold is often cut away or scraped from a cheese surface causing product loss. Advantageously, in one preferred embodiment the present invention provides a shrink film which eliminates or minimizes such losses while utilizing film resins of lower cost than some prior art multilayer shrink films which have thick EVOH containing layers and/or utilizing expensive ionomer resins and/or require relatively thick adhesive layers (each intermediate adhesive layer being  greater than 10-15% of the total film thickness) on either side of the core layer to orient the film. It should also be noted that resins such as anhydride modified polyolefin are relatively expensive and use of thick adhesive layers undesirably increases the cost of multilayer films.
The five layer film of this invention may be manufactured by coextrusion of all layers simultaneously for example as described in U.S. Pat. No. 4,448,792 (Schirmer) or by a coating lamination procedure such as that described in U.S. Pat. No. 3,741,253 (Brax et al.) to form a relatively thick primary multilayer extrudate either as a flat sheet or, preferably, as a tube. This sheet or tube is oriented by stretching at orientation temperatures which are generally below the melting points for the predominant resin comprising each layer oriented. Stretch orientation may be accomplished by various known methods e.g. tentering which is commonly employed to orient sheets, or by the well-known trapped bubble or double bubble technique for orienting tubes as for example described in U.S. Pat. No. 3,456,044 (Pahlke). In this bubble technique an extruded primary tube leaving a tubular extrusion die is cooled, collapsed and then preferably oriented by reheating and inflating to form an expanded secondary bubble which is again cooled and collapsed. Preferred films are biaxially stretched. Transverse direction (TD) orientation is accomplished by the above noted inflation to radially expand the heated film which is cooled to set the film in an expanded form. Machine direction (MD) orientation is preferably accomplished with the use of sets of nip rolls rotating at different speeds to stretch or draw the film tube in the machine direction thereby causing machine direction elongation which is set by cooling. Orientation may be in either or both directions. Preferably, a primary tube is simultaneously biaxially stretched radially (transversely) and longitudinally (machine direction) to produce a multilayer film which is heat shrinkable at temperatures below the melting points of the major polymeric components, e.g. at 90xc2x0 C. or lower. Axially stretched, especially biaxially stretched, films which are xe2x80x9cheat shrinkablexe2x80x9d as that term is used herein have at least 10% unrestrained shrinkage at 90xc2x0 C. (10% in both the machine direction (M.D.) and transverse direction (T.D.) for biaxially stretched films).
The general annealing process by which biaxially stretched heat shrinkable films are heated under controlled tension to reduce or eliminate shrinkage values is well known in the art. If desired, films of the present invention may be annealed to produce lower shrinkage values as desired for the particular temperature. The stretch ratio during orientation should be sufficient to provide a film with a total thickness of between about 1.5 and 4.0 mils. The MD stretch ratio is typically 3-6 and the TD stretch ratio is also typically 3-6. An overall stretch ratio (MD stretch multiplied by TD stretch) of about 9xc3x97-36xc3x97 is suitable.
The preferred method for forming the multilayer film is coextrusion of the primary tube which is then biaxially oriented in a manner similar to that broadly described in the aforementioned U.S. Pat. No. 3,456,044 where the primary tube leaving the die is inflated by admission of a volume of air, cooled, collapsed, and then preferably oriented by reinflating to form a secondary tube termed a xe2x80x9cbubblexe2x80x9d with reheating to the film""s orientation (draw) temperature range. Machine direction (MD) orientation is produced by pulling or drawing the film tube e.g. by utilizing a pair of rollers traveling at different speeds and transverse direction (TD) orientation is obtained by radial bubble expansion. The oriented film is set by rapid cooling. In the following examples, all five layers were coextruded as a primary tube which was cooled upon exiting the die by spraying with tap water. This primary tube was then reheated by radiant heaters with further heating to the draw temperature (also called the orientation temperature) for biaxial orientation accomplished by an air cushion which was itself heated by transverse flow through a heated porous tube concentrically positioned around the moving primary tube. Cooling was accomplished by means of a concentric air ring.
In a preferred process for making films of the present invention, the resins and any additives are introduced to an extruder (generally one extruder per layer) where the resins are melt plastified by heating and then transferred to an extrusion (or coextrusion) die for formation into a tube. Extruder and die temperatures will generally depend upon the particular resin or resin containing mixtures being processed and suitable temperature ranges for commercially available resins are generally known in the art, or are provided in technical bulletins made available by resin manufacturers. Processing temperatures may vary depending upon other process parameters chosen. However, variations are expected which may depend upon such factors as variation of polymer resin selection, use of other resins e.g. by blending or in separate layers in the multilayer film, the manufacturing process used and particular equipment and other process parameters utilized. Actual process parameters including process temperatures are expected to be set by one skilled in the art without undue experimentation in view of the present disclosure.
As generally recognized in the art, resin properties may be further modified by blending two or more resins together and it is contemplated that various resins may be blended into individual layers of the multilayer film or added as additional layers, such resins include ethylene-unsaturated ester copolymer resins, especially vinyl ester copolymers such as EVAs very low density polyethylene (VLDPE), linear low density polyethylene (LLDPE), low density polyethylene (LDPE), high density polyethylene (HDPE), nylons, ionomers, polypropylene or other esters. These resins and others may be mixed by well known methods using commercially available tumblers, mixers or blenders. Also, if desired, well known additives such as processing aids, slip agents, antiblocking agents, pigments, etc., and mixtures thereof may be incorporated into the film.
In some preferred embodiments of the invention it is preferred to crosslink the entire film to broaden the heat sealing range. This is preferably done by irradiation with an electron beam at dosage levels of at least about 2 megarads (MR) and preferably in the range of 3-5 MR, although higher dosages may be employed. Irradiation may be done on the primary tube or after biaxial orientation. The latter, called post-irradiation, is preferred and described in U.S. Pat. No. 4,737,391 (Lustig et al.). An advantage of post-irradiation is that a relatively thin film is treated instead of the relatively thick primary tube, thereby reducing the power requirement for a given treatment level.
Alternatively, crosslinking may be achieved by addition of a chemical crosslinking agent or by use of irradiation in combination with a crosslinking enhancer added to one or more of the layers, as for example described in U.S. Pat. No. 4,055,328 (Evert et al.). The most commonly used cross-linking enhancers are organic peroxides such as trimethylpropane and trimethylacrylate.
It will be seen from the following description that the five layer film of this invention has a very thin EVOH-nylon blend core layer thickness of between about 0.05 and about 0.1 mil yet provides a controlled carbon dioxide (CO2) permeability of between about 75 to 600 cm3/m2 measured at 5xc2x0 C., 0% relative humidity, for 24 hours at 1 atmosphere and relatively low oxygen transmission rate which is preferably less than 800 cm3/m2 at 23xc2x0 C. for 24 hours at 1 atmosphere and 0% relative humidity. This has been accomplished with a substantially thinner barrier layer containing the relatively expensive EVOH than heretofore proposed films of this type. Yet the total thickness of the multilayer film is between about 1.0 and about 4.0 mils, the same as the conventional multilayer oxygen-moisture barrier films currently used in cheese packaging. The relative ratio of EVOH to nylon may be modified to select the desired CO2 permeability. Suitable ranges for high CO2 permeable film may range from 30 to 60% EVOH and 40-70% nylon. Suitable ranges for low CO2 permeable films may range from 60 to 80% EVOH and 20-40% nylon. A mid-range of medium permeabilities may be obtained using from about 30-50% nylon and 70-50% EVOH. This multilayer film of the present invention surprisingly has a very thin nylon: EVOH core layer which is between 0.05 to 0.10 mils thick and less than 10% of the total thickness of the multilayer film, which core layer is connected to surface layers on opposing sides by intermediate adhesive layers each preferably having less than 10% of the total film thickness, and this film having these characteristics may surprisingly be made by using a biaxial stretching process such as that disclosed in U.S. Pat. No. 3,456,044 (Pahlke) with optional irradiative crosslinking after stretching.
This EVOH blend core layer will also control the oxygen permeability of the film. For hard or semi-hard respiring cheese packaging, the oxygen (O2) permeability desirably should be minimized. Typical films will have an O2 permeability of less than about 800 cm3/m2 for a 24 hour period at 1 atmosphere, 0% relative humidity and 23xc2x0 C., and preferably less than 300 cm/m2. For the blends of the present invention the O2 transmission rate (O2GTR) does increase as the CO2 rate increases although not to the same degree. For the desired CO2 permeabilities it has been found than an O2 transmission rate (permeability) of at least about 40 cm3/m2 at 24 hours, 1 atmosphere 0% relative humidity and at 23xc2x0 C. is required, for medium high or higher CO2 permeability the O2GTR will preferably be greater than 75 cm3/m2 for 24 hours at 1 atmosphere, 0% relative humidity and 23xc2x0 C., and for high CO2 permeability films the O2 permeability rate will preferably be at least 150 cm3/m2 or greater.
These former performance levels ( less than 800 cm3/m2 and  less than 300 cm3/m2) are desirable for shrink packaging foodstuffs such as cheeses which are susceptible to contamination with undesirable molds which flourish in the presence of oxygen.
The second and third adhesive layers of this film contain the next most expensive components (relative to the EVOH:nylon blend core layer). Advantageously, films according to the present invention, including heat shrinkable films having high shrinkage values may be in which each adhesive layer comprises only between about 2-10%, preferably less than 5%, eg. about 2.5-3.0% of the multilayer film thickness. Use of a 3% adhesive layer directly adhered to either side of the core layer produces a film which is extremely resistant to delamination and which may be oriented to produce film having high shrinkage of 30% or higher at 90xc2x0 C. or less.
The following are examples and comparative examples given to illustrate the present invention.
Experimental results of the following examples are based on tests similar to the following test methods unless noted otherwise.
Tensile Strength: ASTM D-882, Method A
% Elongation: ASTM D-882. Method A
Haze: ASTM D-1003-52
Gloss: ASTM D-2457, 450 angle
1% Secant Modulus: ASTM D-882, Method A
Oxygen Gas Transmission Rate (O2GTR): ASTM D-3985-81
Water Vapor Transmission Rate (WVTR): ASTM F 1249-90
Elmendorf Tear Strength: ASTM D-1992
Gauge: ASTM D-2103
Melt Index: ASTM D-1238, Condition E (190xc2x0)
Melting point: ASTM D-3418, DSC with 5xc2x0 C./min heating rate
Carbon Dioxide Gas Transmission Rate (CO2GTR): Carbon dioxide gas permeability of film was measured by using an infrared sensor and recorder which is available under the trademark Permatran C-IV by Mocon Testing of Minneapolis, Minn., U.S.A. Each tubular film is cut open to form a flattened sheet. A single thickness of each film sheet is clamped between upper and lower halves of a diffusion cell having dimensions defining a 50 cm2 test area. Carbon dioxide gas (100%) is placed into the upper halve of the diffusion cell. A nitrogen carrier gas, which is free of carbon dioxide, is flushed into the bottom halve of the diffusion cell. This cell is then connected to an infrared sensor and pump creating a closed loop for circulation of the trapped nitrogen carrier gas. The infrared sensor monitors increases in concentration of CO2 as carbon dioxide diffuses through the test film into the closed loop of nitrogen gas, and presents a voltage trace on a strip chart recorder. This trace represents the amount of carbon dioxide diffusing. The carbon dioxide gas transmission rate is derived from the slope of the voltage trace; the instrument having been calibrated by recording voltage changes which correspond to measured amounts of CO2 injected into the instrument.
Shrinkage Values: Shrinkage values are defined to be values obtained by measuring unrestrained shrink at 90xc2x0 C. (or the indicated temperature if different) for five seconds. Four test specimens are cut from a given sample of the film to be tested. The specimens are cut into squares of 10 cm length in the machine direction by 10 cm. length in the transverse direction. Each specimen is completely immersed for 5 seconds in a 90xc2x0 C. (or the indicated temperature if different) water bath (or silicone oil if the test temperature is greater than 100xc2x0 C.). The specimen is then removed from the bath and the distance between the ends of the shrunken specimen is measured for both the M.D. and T.D. directions. The difference in the measured distance for the shrunken specimen and the original 10 cm. side is multiplied by ten to obtain the percent of shrinkage for the specimen in each direction. The shrinkage for the four specimens is averaged for the M.D. shrinkage value of the given film sample, and the shrinkage for the four specimens is averaged for the TD shrinkage value.
Shrink Force: The shrink force of a film is that force or stress required to prevent shrinkage of the film and was determined from film samples taken from each film. Four film samples were cut 1xe2x80x3 (2.54 cm) wide by 7xe2x80x3 (17.8 cm) long in the machine direction and 1xe2x80x3 (2.54 cm) wide by 7xe2x80x3 (17.8 cm) long in the traverse direction. The average thickness of the film samples was determined and recorded and a strip chart recorder was calibrated at 0 gram and at 1,000 grams full scale load. Each film sample was then secured between the two clamps spaced 10 cm apart. One clamp is in a fixed position and the other is connected to a strain gauge transducer. The secured film sample and clamps were then immersed in a silicone oil bath maintained at a constant, elevated temperature for a period of five seconds. During this time, the force in grams at the elevated temperature was read from the strip chart and this reading was recorded. At the end of this time, the film sample was removed from the bath and allowed to cool to room temperature whereupon the force in grams at room temperature was also read from the strip chart and recorded. The shrink force for the film sample was then determined from the following equation wherein the results is obtained in grams per mil of film thickness (g/mil):
Shrink Force (g/mil)=F/T 
wherein F is the force in grams and T is the average thickness of the film samples in mils.
Impulse Seal Range:
The impulse sealing range test determines the acceptable voltage ranges for impulse sealing plastic films. A Sentinel Model 12-12AS laboratory sealer manufactured by Packaging Industries Group, Inc., Hyannis Mass., U.S.A. was used. This impulse sealer is equipped with a replacement sealing ribbon for a Multivac AG100 brand packaging machine. The ribbon is available from Koch Supplies of Kansas City, Mo. In this test, two four inch wide (T.D. direction) samples are cut from a tubular film. The impulse sealer is equipped with controls for coolant flow, impulse voltage and time, and seal bar pressure. These controls except for impulse voltage are set at the following conditions:
One of the samples is folded in half for use in determining a minimum sealing voltage. This folding simulates folding which may inadvertently occur during conventional bag sealing operations. The folded sample which now has four sheets or portions of film (hereinafter referred to as xe2x80x9csheet portionsxe2x80x9d) is placed into the sealer and by trial and error the minimum voltage to seal the bottom two sheet portions to each other was determined.
The maximum voltage is then determined for a sample having two sheet portions by placing it in the sealer and then activating the seal bar. The film sample is manually pulled with about 0.5 lbs. of force and the voltage which does not cause burn through or significant distortion of the seal is determined.
Gassing Test: The Gassing Test is an evaluation of film adherence to a vacuum packaged respiring natural cheese. In this test a rectangular block of respiring natural cheese is vacuum packaged in a film which is hermetically sealed. Due to microbiological activity many natural cheeses such as emmental (swiss) respire or give off CO2. Therefore, over time CO2 gas will build up in a sealed film package unless the film is permeable to CO2. This build up of gas will inflate the sealed package if the rate of CO2 generation is greater than the rate of permeability CO2 through the film wall. The amount of inflation will depend upon both the gas production rate and the film permeability or gas transmission rate. Film adherence to the surface of the packaged cheese is visually evaluated and given a numerical value from 0 to 10, with greater value numbers indicating less adherence and more ballooning. 0=complete film adherence to the cheese product. 5=film ballooned on flat sides, but product corners and edges in contact with film, 7=film ballooned away from all surfaces except corners, 10=complete film ballooning away from all product surfaces including flat sides, edges and corners. The same person evaluates all packages in a test to ensure accuracy. Evaluations are made over time and the time elapsed for each evaluation is reported, generally in days. The packaged cheeses are all held at about 35xc2x0 F. (xcx9c2xc2x0 C.) over the evaluation period. Standard deviation for multiple examples may be reported as may differences (xcex94) in values for particular packages from one test evaluation over time to the next evaluation.
Following are examples and comparative examples given to illustrate the invention.
In all the following examples, unless otherwise indicated, the film compositions were produced generally utilizing the apparatus and method described in U.S. Pat. No. 3,456,044 (Pahlke) which describes a coextrusion type of double bubble method and in further accordance with the detailed description above. All percentages are by weight unless indicated otherwise.